Although coal is a useful energy source with an abundant reserve, it is restricted in its field of application as compared with petroleum and natural gas because it is solid and has a high ash content. However, when coal is transformed into gas or liquid, it can be used in much wider fields of application and can he a more useful energy source. Accordingly, technologies for fluidizing coal are being developed in various countries.
Under such circumstances, particularly the combined coal gasification-power generation system is attracting attention as a power generation process for the next generation. The "combined coal gasificationpower generation system" is a system in which a combustible gas of high temperature is produced in a coal gasification furnace, steam is formed by recovering the sensible heat of the gas produced above, a steam turbine is driven by the steam formed, and concurrently a gas turbine is driven by the gasified combustible gas. This system can provide an improvement of several percent in power generation efficiency as compared with prior systems comprising a steam turbine alone. The coal gasification furnace is a principal component of the combined coal gasification-power generation system, and hence many companies are conducting the research and development of the furnace.
In coal gasification, attempts are being made to convert coal into gas in a high efficiency by use of such forms as a fixed bed, fluidized bed, and jet stream bed. An important problem for each form is how, during the gasification, to separate ash in the coal effectively from the produced gas and to remove the ash as a non-polluting substance from the gasification furnace.
A useful method for withdrawing ash in coal as a non-polluting substance comprises melting the ash, covering the surface of the ash with components contained in the ash itself and chaning the property of the surface into that of glass. Such a method of treating ash enables confinement of harmful metals contained in the coal ash within the ash particle. Thus, it is an effective method of treating ash from the viewpoint of environmental hygiene because no harmful metal is leached out of the ash with water etc. when the coal ash is employed, for example, in land reclamation. Further, this method of treatment can increase the density of ash several times in comparison with that of fly ash, which is the ash discharged from a prior thermal power generation boiler using pulverized coal, and hence can drastically decrease the volume of ash. Thus, it affords a great advantage in handling of ash. Accordingly, gasification furnaces using a fixed bed or jet stream bed mentioned above also adopt a structure wherein molten coal ash, namely slag, is stored in a slag tap positioned at the bottom of the furnace and is further dropped into a slag cooling chamber positioned below.
When the slag cannot be dropped stably from the slag tap to the slag cooling chamber, various problems occur. For example when the slag cannot be dropped stationarily, the ash in coal will scatter in large quantities into the downstream gas of the gasification furnace. In dust collectors, such as cyclones and bag filters, provided in the downstream, such problems as excessive differential pressure and clogging will take place owing to the intrusion of dust exceeding the design quantity. In the worst case, the emergency shut down of the gasification furnace becomes necessary owing to clogging of pipe lines.
Further, when the slag tap is clogged, slag will stay at the furnace bottom. If the operation of the furnace is still continued under such conditions, the slag will clog the outlet of the lower stage burner, inevitably resulting in stoppage of the operation. In order to start the operation again, it is necessary to dismantle the furnace, repair the furnace bottom part and replace the slag tap. In the worst case, the furnace becomes unrestorable.
Since coal is in general different in ash content and ash composition depending on the place of production, the melting temperature of the ash is also varied. Some ash melts at as low a temperature as about 1200.degree. C. and some other does not melt even at 1600.degree. C. or above.
Accordingly, it is one of the important problems for coal gasification to develop a furnace in which the stable dropping of molten slag is possible even when various kinds of coal with various ash compositions are used.
Regarding the slag tap, descriptions are found, for example, in Japanese Patent Application Kokai (Laid-open) Nos. 54,395/80 and 58,703/79. The former relates to the structure and material of a slag dropping part, and the latter relates to the structure of a burner used for heating the dropping part. These technologies aim at stable dropping of slag. The former is a method to be used for coal whose ash has a low melting point, and causes difficulty in dropping of ash having a high melting point. The latter method is effective also for ash of a high melting point because a heating burner is provided therein. In this method, however, the gas ring and the air ring of the heater provided at the dropping part are arranged in two stages and hence, in long time operation, they are subjected to thermal strain and the flame will deviate from the proper position for dropping the slag. Further, since the direction of gas flow and that of dropping slag is opposite, a smooth dropping is difficult to obtain. Further, in Japanese Patent Application Kokai (Laid-open) No. 76,506/82, the gasification furnace is constructed in multistage regarding the heating part as one furnace and heating in the furnace positioned under the slag tap is effected by use of a heavy oil of relatively low ash content.
The drawback of these methods lies in the use of a burner as an auxiliary heating means. Surely it is necessary to keep the slag dropping device at a temperature not lower than the melting point of slag in order to secure smooth dropping of the slag. For this purpose it is the most suitable to use a burner as an auxiliary heating means of high heat efficiency.
However, the auxiliary fuel to be used in heating the slag tap lower part should be an expensive, ash-free clean fuel, which is economically unadvantageous. Various studies have been made to obviate this defect. Resulting proposals mainly relate to fuels to be used. For example, the use of produced gas as the auxiliary fuel has been proposed. Since a coal gasification furnace produces by nature a combustible gas, the proposed method is effective because it needs no other fuel. However, in recycling the produced gas, the gas must be pressurized before being supplied to the gasification furnace, which results in complicating the apparatus. Further, since a high temperature gas is cooled and purified before use, heat loss is serious. An example of using coal itself as the auxiliary fuel has been proposed in Japanese Patent Application Kokai (Laid-open) No. 76,302/76. In this method, however, molten ash formed in the combustion of coal adheres to the lower part of the slag tap, causing an operational problem.
In any case, when a burner is provided, a fuel supply device, a control device etc. attendant thereon become necessary, making the system very complicated. Further, although recent gasification furnaces tend to aim at operation at higher pressures to increase efficiency or to increase capacity, many technical problems remain yet under high pressure conditions with regard the ignition and control of the burner. Burners to be used at high pressures are still in a developmental stage, and a reliable technology has not been established yet.
The ultimate form required for a slag tap is a slag tap heated by the heat of the furnace itself. When viewed from such a point, the hitherto proposed methods may be divided roughly into two groups. One is to heat the slag tap by passing the produced gas of high temperature in the furnace through the slag tap, namely the so-called downblow method. The other is to transfer the heat in the gasification furnace to the slag tap by means of heat transmission.
A typical example of heating the slag tap by passing the produced gas of high temperature in the furnace therethrough is found in a Texaco-type furnace. A "Texaco-type furnace" is a furnace in which the produced gas is withdrawn directly from the slag tap disposed at the bottom part of the gasification furnace. Accordingly, the clogging of the slag tap is not likely to occur. However, when the gas flow is downward, the relative velocity between the coal particles and the gasifying agent is small, resulting in a decreased gasification efficiency. Further, although it is essentially desirable at a slag tap to separate molten slag from produced gas and withdraw the slag alone, the total amount of the produced gas is withdrawn through the slag tap in this furnace and hence slag is entrained by the gas, resulting in poor efficiency in slag separation.
One example wherein the gas flow is upward and part of the produced gas is withdrawn from the slag tap is disclosed in Japanese Patent Application Kokai (Laid-open) No. 232,173/84. However, this method has a problem regarding the material of the pipe through which a high temperature gas is passed. Further, in order that a sufficient suction effect of an orifice may be obtained, the gas velocity at the furnace outlet should be 100 m/s or more, giving rise to fear of the abrasion of the material used in the furnace outlet part.